The Specific Aim of the Program Project is to clarify the role of deficient reinnervation in the poor quality of regeneration of muscle grafted in old compared with young animals. Project #2 will contribute to the Specific Aims of the Program Project through measurements of structural and functional properties. Structural properties include: number of motor units, mean total fiber cross-sectional area of single motor units, concentration of contractile protein, and myosin isoform composition of single fibers. Functional properties are: absolute and normalized maximum force and maximum power, and fatigability. The EDL muscles and grafts will be obtained from 4 to 6 month young and 26 to 28 month old male rats. Muscles will be autografted, or cross-age transplanted with nerves implanted or intact. Project #2 will test two of the Group Hypotheses that: A. Impairments in structural and functional properties of grafts in old compared with young hosts are related to a deficiency in reinnervation; and B. In comparing muscle fiber regeneration in young and old hosts, two stages can be identified in relation to the time of innervation of the graft-1) a preinnervation stage, in which the course of muscle fiber regeneration is not significantly different between young and old grafts, and 2) a postinnervation stage in which muscle fibers regenerating in old animals show deficits compared with those regenerating in young animals. Group Hypotheses A and B will be supported if Specific Hypotheses regarding cross-age transplanted grafts and nerve-intact autografts are upheld. Specific Hypotheses will be upheld if grafts in old compared to young rats demonstrate: A-1. a delayed reinnervation; A-2. fewer motor units; and B-2. a post innervation stage in muscle regeneration characterized by deficits in the properties of motor units and single fibers. Data on functional properties will be related to data on structural properties to test, formulate and retest hypotheses regarding the mechanisms responsible for the poor regeneration of muscle in old hosts (to be done with P-1). The impairments in grafts in old compared with young animals are not qualitatively different, but are of 2- to 3-fold greater magnitude than those observed between old and young control animals. Consequently, data from the cross-transplantation model offers a high probability of clarifying the causes of the 30 to 40% decrease in the muscle mass and strength of skeletal muscles of old compared with young human beings.